Booking.com isn’t the only website giving law-breaking residential hotel operators access to the tourist market. At Hostelworld.com, you can book a unit here at the Globetrotter Inn that is an illegally converted residential hotel room with six beds crammed in it, a likely violation of city occupancy code. But booking.com is the only travel site with an office in San Francisco that posts illegal tourist hotels. Here is why that matters.

As BeyondChron wrote yesterday, illegal hotel conversions are booming again – thanks to European tourists who value location and price over opulent lobbies or large bedrooms, operators who see dollar signs in renting their rooms illegally by the night to these tourists, and Internet travel sites who earn sales commissions from each illegal transaction. But not every Internet travel site offers illegal tourist rooms. Expedia.com (which owns hotels.com), orbitz.com and Travelocity.com do not allow you to book these rooms. So why is booking.com partnering with these illegal businesses?

Holding Internet travel sites responsible for the content of a third-party’s postings is difficult to do. Take the case of Hostelworld.com. For one, the company is based in Ireland and thus can make a credible claim of not knowing San Francisco’s Hotel Conversion Ordinance. Second, because it’s not located in each of the markets that it advertises, websites like Hostelworld.com holds a business agreement with each operator which puts the liability for the posting’s content squarely on the operator.

But Booking.com is located in San Francisco – so it’s not as credible to claim ignorance of local law, which also means its local partner agreements don’t shield it from responsibility as well as those agreements made in other locations. And as a local business, they could be more amenable to pressure from housing activists than companies located overseas.

If the City is serious about stopping illegal hotel conversions, the City Attorney and District Attorney should review Booking.com’s business practices. The stakes are high. It would take years and tens of millions of dollars for the City to replace the 1,000 affordable housing units it could lose tomorrow to illegal conversion.

Not to mention the fact that each illegal booking takes money away from law-abiding operators and serves as an incentive to other residential operators who operate legally but are tempted by the larger profit margins of the tourist trade.